Metronidazole comes in several forms. These include an oral tablet, an oral capsule, a cream, gel, and lotion you apply to your skin, and a vaginal gel. It also comes as an injectable medication given by a healthcare provider.

Metronidazole oral tablets are used to treat infections caused by bacteria or parasites.

The metronidazole immediate-release tablet and extended-release tablet are prescription drugs. They’re both taken by mouth. These tablets are available as the brand-name drugs Flagyl (immediate-release) and Flagyl ER (extended-release).

Immediate-release drugs are released into the body right away. Extended-release drugs are released into the body slowly over time.

Both the immediate-release and extended-release tablets are available as generic drugs. Generic drugs usually cost less than the brand-name version. In some cases, they may not be available in all strengths or forms as the brand-name drug.

Why it’s used

Metronidazole immediate-release oral tablets are used to treat many infections caused by bacteria or parasites. These include infections that occur in the gastrointestinal tract or reproductive system such as amebiasis and trichomoniasis. Metronidazole extended-release oral tablets are used to treat vaginal infections in women.

Metronidazole may be used as part of a combination therapy. This means you may need to take it with other medications.

How it works

Metronidazole belongs to a class of drugs called nitroimidazole antimicrobials. A class of drugs is a group of medications that work in a similar way. These drugs are often used to treat similar conditions.

Antimicrobials are drugs used to treat infections. Nitroimidazole antimicrobials treat infections caused by bacteria and other organisms called protozoa. Metronidazole tablets work by killing the bacteria or other organism that’s causing the infection. This relieves the infection.

Metronidazole oral tablet doesn’t cause drowsiness, but it can cause other side effects.

More common side effects

The more common side effects that can occur include:

headache

loss of appetite

nausea or vomiting

diarrhea

heartburn

cramping in your stomach area

constipation

metallic taste

If these effects are mild, they may go away within a few days or a couple of weeks. If they’re more severe or don’t go away, talk to your doctor or pharmacist.

Serious side effects

Call your doctor right away if you have serious side effects. Call 911 if your symptoms feel life-threatening or if you think you’re having a medical emergency. Serious side effects can include:

Nervous system effects, including seizures and encephalopathy (abnormal brain function). Symptoms can include:

convulsions (sudden movements caused by tightening of your muscles)

dizziness

headache

confusion

ataxia (loss of control of body movements)

Disclaimer: Our goal is to provide you with the most relevant and current information. However, because drugs affect each person differently, we cannot guarantee that this information includes all possible side effects. This information is not a substitute for medical advice. Always discuss possible side effects with a healthcare provider who knows your medical history.

Side effects for men vs. women

For the most part, the side effects of metronidazole for men and women are the same. The only real differences in side effects affect women. For instance, metronidazole increases the risk of yeast infections, which occur much more often in women. Also, metronidazole can cause vaginal irritation and discharge.

Metronidazole oral tablet can interact with other medications, vitamins, or herbs you may be taking. An interaction is when a substance changes the way a drug works. This can be harmful or prevent the drug from working well.

To help avoid interactions, your doctor should manage all of your medications carefully. Be sure to tell your doctor about all medications, vitamins, or herbs you’re taking. To find out how this drug might interact with something else you’re taking, talk to your doctor or pharmacist.

Examples of drugs that can cause interactions with metronidazole are listed below.

Drugs you should not take with metronidazole

Disulfiram: Do not take disulfiram with metronidazole. Doing so can cause dangerous effects in your body. Using it with metronidazole can cause psychotic reactions. Symptoms can include:

confusion

hallucinations (seeing or hearing things that aren’t real)

delusions (believing things that aren’t real)

Do not take metronidazole if you’ve taken disulfiram in the last two weeks.

Interactions that increase your risk of side effects

Taking metronidazole with certain medications raises your risk of side effects. This is because the amount of either drug may be increased in your body. Examples include:

Lithium: Increased side effects are related to raised lithium levels. Your doctor should monitor your lithium levels if you take these drugs together.

Warfarin or other blood thinners: Increased side effects of these drugs include a higher risk of bleeding.

Busulfan: If possible, you should avoid taking busulfan with metronidazole. If you do take these drugs together, your doctor may check the amount of busulfan in your body more often.

Cimetidine: Taking cimetidine with metronidazole may result in higher metronidazole levels in your body and increased side effects.

Phenytoin or phenobarbital: Taking one of these drugs with metronidazole may result in reduced metronidazole levels in your body. This could prevent metronidazole from curing your infection.

Disclaimer: Our goal is to provide you with the most relevant and current information. However, because drugs interact differently in each person, we cannot guarantee that this information includes all possible interactions. This information is not a substitute for medical advice. Always speak with your healthcare provider about possible interactions with all prescription drugs, vitamins, herbs and supplements, and over-the-counter drugs that you are taking.

All possible dosages and drug forms may not be included here. Your dosage, drug form, and how often you take the drug will depend on:

your age

the condition being treated

the severity of your condition

other medical conditions you have

how you react to the first dose

For bacterial and protozoal infections

Generic: Metronidazole

Form: immediate-release oral tablet

Strengths: 250 mg, 500 mg

Brand: Flagyl

Form: immediate-release oral tablet

Strengths: 250 mg, 500 mg

Adult dosage (ages 18–64 years)

Your dosage and length of treatment depend on your infection type.

Bacterial infections:

Typical dosage: 500 mg four times per day for 7–10 days. However, some infections may require longer treatment.

Maximum dosage: 4 g per day.

Amoebic infections:

Typical dosage: 500 mg or 750 mg three times per day for 5–10 days.

Trichomoniasis:

Typical dosage: Either 2 grams (g) as a single dose or two divided doses of 1 g each on a single day, or 250 mg three times per day for 7 days.

Child dosage (ages 0–17 years)

Amoebic infections:

Typical dosage: 35–50 mg/kg of bodyweight per day given in three divided doses for 10 days.

Senior dosage (ages 65 years and older)

The kidneys and liver of older adults may not work as well as they used to. This can cause your body to process drugs more slowly. As a result, more of a drug stays in your body for a longer time. This increases your risk of side effects. Your doctor may start you on a lowered dose or a different medication schedule. This can help keep levels of this drug from building up too much in your body.

For bacterial vaginosis

Generic: Metronidazole

Form: extended-release oral tablet

Strengths: 750 mg

Brand: Flagyl ER

Form: extended-release oral tablet

Strength: 750 mg

Adult dosage (ages 18–64 years)

Typical dosage: 750 mg per day for 7 days.

Senior dosage (ages 65 years and older)

The kidneys and liver of older adults may not work as well as they used to. This can cause your body to process drugs more slowly. As a result, more of a drug stays in your body for a longer time. This increases your risk of side effects.

Your doctor may start you on a lowered dose or a different medication schedule. This can help keep levels of this drug from building up too much in your body.

FDA warnings

Metronidazole oral tablets and capsules have a black box warning. This is the most serious warning from the Food and Drug Administration (FDA). A black box warning indicates drug effects that may be dangerous.

Cancer warning: Cancer was found in some animals during testing with metronidazole. There may be a similar risk in humans. Because of this risk, metronidazole should only be used to treat conditions as approved by the FDA.

Usage warning: To reduce the occurrence of drug-resistant bacteria, metronidazole should only be used to treat or prevent infections that are proven or strongly suspected to be caused by bacteria.

Allergy warning

Metronidazole can cause a severe allergic reaction or hypersensitivity. Symptoms can include:

hives (itchy welts)

red rash

peeling or sloughing of your skin

wheezing

fever

chills

If you have an allergic reaction, call your doctor or local poison control center right away. If your symptoms are severe, call 911 or go to the nearest emergency room.

Don’t take this drug again if you’ve ever had an allergic reaction to it. In some cases, taking it again could lead to death.

Alcohol interaction warning

You should stop using drinks that contain alcohol at least three days before starting metronidazole. Also avoid alcohol for three more days after you stop treatment with this drug.

This is because alcohol can cause side effects when used with metronidazole. These include:

nausea

vomiting

abdominal cramps

headaches

flushing (sudden redness and warmth in your face)

If you drink alcohol, talk to your doctor about whether this drug is safe for you.

Warnings for certain groups

For people with liver disease: Your liver helps process this drug. If you have severe liver disease, your liver may process this drug more slowly. This would increase the amount of the drug in your body and raise your risk of side effects. Your doctor may lower your dosage of metronidazole or have you take it less often.

For people with kidney disease: Your kidneys help clear this drug from your body. If you have severe kidney disease, your kidneys may process this drug more slowly. This increases the amount of the drug in your body and raises your risk of side effects. Your doctor may lower your dose of metronidazole or have you take it less often.

For pregnant women: Metronidazole is a category B pregnancy drug. That means two things:

Studies of the drug in pregnant animals have not shown a risk to the fetus.

There aren’t enough studies done in pregnant women to show if the drug poses a risk.

Talk to your doctor if you’re pregnant or planning to become pregnant. Metronidazole should not be taken during the first trimester of pregnancy. For the second and third trimesters, this drug should be used only if the potential benefit justifies the potential risk.

For women who are breastfeeding: Metronidazole may pass into breast milk and cause side effects in a child who is breastfed. Talk to your doctor if you breastfeed your child. You may need to decide whether to stop breastfeeding or stop taking this medication.

For seniors: The kidneys and liver of older adults may not work as well as they used to. This can cause your body to process drugs more slowly. As a result, more of a drug stays in your body for a longer time. This increases your risk of side effects.

Metronidazole tablets are used for short-term treatment. They come with risks if you don’t take them as prescribed.

If you stop taking the drug suddenly or don’t take it at all: Your infection may not improve and may get worse.

If you miss doses or don’t take the drug on schedule: Your medication may not work as well or may stop working completely. For this drug to work well, a certain amount needs to be in your body at all times.

If you take too much: You could have dangerous levels of the drug in your body. Symptoms of an overdose can include increased side effects, such as:

headache

loss of appetite

nausea or vomiting

diarrhea

heartburn

cramping in your stomach area

constipation

metallic taste

If you think you’ve taken too much of this drug, call your doctor or seek guidance from the American Association of Poison Control Centers at 1-800-222-1222 or through their online tool. But if your symptoms are severe, call 911 or go to the nearest emergency room right away.

What to do if you miss a dose: Take your dose as soon as you remember. But if you remember just a few hours before your next scheduled dose, take only one dose. Never try to catch up by taking two doses at once. This could result in dangerous side effects.

How to tell if the drug is working: Your symptoms of infection should improve.

General

You can take the immediate-release tablets with or without food. Taking them with food may help reduce upset stomach.

Do not take the extended-release tablets with food. You should take them at least 1 hour before a meal or 2 hours after a meal.

You can cut or crush the immediate-release tablets. However, do not cut or crush the extended-release tablets.

Take metronidazole at the time(s) recommended by your doctor.

Storage

Store this drug at a temperature below 86°F (30°C).

Keep it away from light.

Don’t store this medication in moist or damp areas, such as bathrooms.

Refills

A prescription for this medication is refillable. You should not need a new prescription for this medication to be refilled. Your doctor will write the number of refills authorized on your prescription.

Travel

When traveling with your medication, follow these tips:

Carry your medication with you. When flying, never put it into a checked bag. Keep it in your carry-on bag.

Disclaimer:Medical News Today has made every effort to make certain that all information is factually correct, comprehensive, and up-to-date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or other healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.